Conventionally, a variety of power transmission devices are known, which transmit rotational driving force output from a drive source to a target device to be driven.
Patent Literature 1 discloses, as a power transmission device, an electromagnetic clutch that transmits a rotational driving force outputted from an engine to a compressor of a refrigeration cycle. The electromagnetic clutch includes a cover that covers the outer circumference side of the electromagnetic clutch to suppress foreign objects such as water and dusts from intruding into a frictional surface of the clutch or a bearing forming an attachment portion to the compressor.